Add struck arrows

[MDLC01]

This PR adds struck arrows, as well as twohead tailed arrows.

For singly struck arrows, I chose the .struck modifier, which is consistent with its use in parallel.struck in contrast to parallel.not. Unicode also defines doubly struck arrows. For those, an additional modifier needs to be created, and i am unsure of the name to use. Maybe .doublestruck, or .dstruck? I marked the PR as a draft for this reason.

[Enivex]

struck.double or struck.twice?

[T0mstone]

struck.double or struck.twice?

arrow.double is already something else. This is a case where our commutative modifiers are a limitation, since the "double" would be a modifier that applies to another modifier.

Edit: Unless you were suggesting making struck its own symbol, but even then that couldn't distinguish between ⤃ and ⇻.

[Enivex]

Are modifiers commutative though?

That seems like a very bad idea

[T0mstone]

Are modifiers commutative though?

Yesn't.
They are in typst, that's how circle.filled.big is the same as circle.big.filled.
For codex by itself, modifiers don't exist. We only have variants until/unless #46 is merged.

[knuesel]

Maybe .struck-twice ?

[MDLC01]

The dash would prevent it from being easily usable in math

[knuesel]

Ah right, I confusedly imagined this would be OK after the change to math argument parsing in Typst 0.13 but obviously not!

In favor of dstruck: we could consistently use a d prefix for double modifiers, like we use n for negation in .neq, .neqiv, .ntilde.

Or we could aim for consistency with arrow.r.twohead and use arrow.r.twostroke (as in the Unicode name Rightwards Arrow with Double Vertical Stroke). Then it would make sense to rename arrow.r.struck to arrow.r.stroke...

[MDLC01]

arrow.r.stroke is too similar to arrow.r.stroked imo.

[MDLC01]

I went with .dstruck. As mentioned above, it creates a consistency with .neq. If anything, .twohead is the outlier here, although I don't think it would be necessary to change it.